This invention relates to radiation detectors and particularly to neutron radiation detectors for measuring the presence of uranium in ore bodies.
In the exploration and mining of uranium from naturally occurring ore bodies, it is necessary to determine the location and quantity of the ore in order to determine if mining the particular ore body is economically feasible. There are many methods known in the art for measuring ore bodies, one of which is well logging in which boreholes are drilled in the ore body and electronic instruments are moved through the borehole. The electronic instruments detect radiation from the ore body which gives an indication of the amount of radioactive material present in the ore body. However, unlike uranium, not all radioactive material is of a fissionable nature and, therefore, not desirable to mine. For example, radium which is a daughter product of uranium is radioactive but is not fissionable. Furthermore, while radium and uranium are often found together, geological or natural chemical processes may separate them so that a radiation indication may result from the presence of radium and not be an indication of the presence of uranium. Therefore, it is important to be able to determine the location of fissionable isotopes such as uranium by utilizing mechanisms that can distinguish between the radiation produced by fissionable isotopes and the radiation produced by non-fissionable isotopes.
In U.S. Pat. No. 3,686,503, entitled "In-Situ Assaying For Uranium In Rock Formations", issued Aug. 22, 1972 by W. W. Givens et al. there is described a method for quantitatively measuring uranium ore grade. The method employs a tool containing a source of neutrons and a neutron detector that is placed in a borehole at the level of a formation of interest. The source is operated cyclically to irradiate a zone in the formation with neutrons. Neutrons resulting from the irradiation of the zone in the formation are detected and recorded to obtain a record of delayed neutrons emitted as a result of neutron fission of uranium. Recording takes place within each cycle beginning at a time period after the source neutrons have died away via absorption in the formation. Recording the number of neutrons emitted as a result of the fission process gives an indication of the quantity of uranium present in the formation.
While there exist methods of measuring delayed neutrons to give an indication of the uranium content of the ore body, these methods can suffer from inefficient use of the neutron source by not detecting a large fraction of the radioactivity induced by neutrons because the delayed neutron detectors only span a short length of activated zone while those of longer lengths do not provide good spatial resolution. Therefore, a neutron activation probe is needed that is capable of accurately measuring delayed neutrons while making efficient use of the neutron source to assay uranium ore and yet relating the results to a specific location in the formation.